The present invention relates to a distance/speed meter and distance/speed measuring method which measure at least the distance to a measurement target or the speed of the measurement target by using optical interference.
Distance measurement by a laser using optical interference does not disturb a measurement target because of noncontact measurement, and has been used for a long time as a high-accuracy measuring method. Recently, attempts have been made to use a semiconductor laser as a light measurement light source to achieve a reduction in apparatus size. A typical example of such an apparatus is an apparatus using an FM heterodyne interferometer. This apparatus can measure a relatively long distance with high accuracy, but has a drawback of a complicated optical system because of the use of an interferometer outside a semiconductor laser.
In contrast to this, measuring instruments have been proposed, which use the interference between output light from a semiconductor laser and return light from a measurement target inside the laser (self-mixing effect) in, for example, reference 1 (Tadashi Ueda, Jun Yamada, and Susumu Shitoh, “Distance Meter Using Self-Coupled Effect of Semiconductor Laser”, Papers for 1994 Tokai-Section Joint Conference of the 8 Institutes of Electrical and Related Engineers), reference 2 (Jun Yamada, Susumu Shitoh, Norio Tuda, and Tadashi Ueda, “Study of Compact Distance Meter by Self-Coupled Effect of Laser Diode”, Bulletin of Aichi Institute of Technology, Vol. 31B, pp. 35-42, 1996), and reference 3 (Guido Giuliani, Michele Norgia, Silvano Donati and Thierry Bosch, “Laser diode self-mixing technique for sensing applications”, JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS, pp. 283-294, 2002).
In such a self-mixing type laser measuring instrument, a photodiode built-in semiconductor laser has light-emitting, interference, and light-receiving functions at the same time, and hence allows great simplification of an external interference optical system. A sensor unit therefore comprises only a semiconductor laser and a lens, and becomes smaller than conventional sensor units. This instrument also has a characteristic feature that its distance measurement range is wider than that of triangulation.
FIG. 39 shows a complex cavity model of an FP type (Fabry-Perot type) semiconductor laser. Part of reflected light from a measurement target 104 tends to return into an oscillation area. Slight return light mixes with laser light inside a semiconductor laser cavity 101, resulting in unstable operation and noise (complex cavity noise or return light noise). Even a very small amount of return light relative to output light causes a noticeable change in the characteristics of the semiconductor laser. Such a phenomenon is not limited to a Fabry-Perot type (to be referred to as an FP type) semiconductor laser, and also occurs in other types of semiconductor lasers such as a vertical cavity surface emitting laser (to be referred to as a VCSEL type hereinafter) and a distributed feedback laser type (to be referred to as a DFB laser type).
Let λ be the oscillation wavelength of the laser and L be the distance from a cleavage plane 102 near the measurement target 104 to the measurement target 104. In this case, when the following resonance condition is satisfied, return light and laser light in the cavity 101 strengthen each other. Consequently, the laser power slightly increases.L=qλ/2  (1)where q is an integer. This phenomenon can be sufficiently observed even with very weak scattered light from the measurement target 104 when an amplifying action occurs as the apparent reflectance inside the semiconductor laser cavity 101 increases.
A semiconductor laser emits laser light having different frequencies in accordance with the magnitude of injection current. This laser therefore allows direct modulation of the oscillation frequency by using an injection current without requiring any external modulator when an oscillation frequency is modulated. FIG. 40 shows the relationship between the oscillation wavelength and the output waveform of a photodiode 103 when the oscillation wavelength of the semiconductor laser is changed at a predetermined rate. When L=qλ/2 indicated in equation (1) is satisfied, the phase difference between return light and laser light inside the cavity 101 becomes 0° (in phase), and the return light and the laser light inside the cavity 101 strengthen each other most. When L=qλ/2+λ/4, the phase difference becomes 180° (in opposite phase), and the return light and the laser light inside the cavity 101 weaken each other most. As the oscillation wavelength of the semiconductor laser is changed, therefore, the laser power increases and decreases alternately and repeatedly. When the laser power is detected at this time by the photodiode 103 provided in the cavity 101, a stepwise waveform having a constant period like that shown in FIG. 40 is obtained. Such a waveform is generally called an interference fringe.
Each of the elements of this stepwise waveform, i.e., the interference fringe, is called a mode hop pulse (to be referred to as an MHP hereinafter). MHP is a phenomenon different from a mode hopping phenomenon. Assume that the distance to the measurement target 104 is represented by L1, and the number of MHPs is 10. In this case, as the distance decreases to L2 which is ½ of L1, the number of MHPs becomes five. That is, as the oscillation wavelength of the semiconductor laser changes in a predetermined time, the number of MHPs changes in proportion to the measurement distance. Therefore, detecting MHPs by the photodiode 103 and measuring the frequency of MHPs can easily measure the distance.
A self-mixing type laser measuring instrument allows great simplification of an external interference optical system outside a cavity, and hence can achieve downsizing. In addition, this instrument requires no high-speed circuit and is robust against disturbance light. In addition, the instrument can operate even with very weak return light from a measurement target, and hence is not influenced by the reflectance of the measurement target. That is, the instrument can be used for any types of measurement targets. However, conventional interference type distance meters including self-mixing type distance meters cannot measure distances to moving measurement targets, even though they can measure distances to stationary measurement targets.
The present inventor has therefore proposed a distance/speed meter which can measure the speed of a measurement target as well as the distance to a stationary measurement target, as disclosed in reference 4 (Japanese Patent Laid-Open No. 2006-313080). FIG. 41 shows the arrangement of this distance/speed meter. The distance/speed meter in FIG. 41 includes a semiconductor laser 201 which applies a laser beam to a measurement target, a photodiode 202 which converts an optical output from the semiconductor laser 201 into an electrical signal, a lens 203 which focuses light from the semiconductor laser 201 to apply it to a measurement target 210 and also focuses return light from the measurement target 210 to make it strike the semiconductor laser 201, a laser driver 204 which causes the semiconductor laser 201 to alternately repeat the first oscillation interval in which the oscillation wavelength of the semiconductor laser 201 continuously increases and the second oscillation interval in which the oscillation wavelength continuously decreases, a current/voltage conversion amplifier 205 which converts an output current from the photodiode 202 into a voltage and amplifies it, a signal extraction circuit 206 which calculates the second-order differential of an output voltage from the current/voltage conversion amplifier 205, a counting circuit 207 which counts the number of MHPs contained in an output voltage from the signal extraction circuit 206, a computing device 208 which calculates the distance to the measurement target 210 and the speed of the measurement target 210, and a display device 209 which displays the calculation result obtained by the computing device 208.
The laser driver 204 supplies a triangular wave driving current, which repeatedly increases and decreases at a constant change rate with respect to time, as an injection current and supplies it to the semiconductor laser 201. With this operation, the semiconductor laser 201 is driven to alternately repeat the first oscillation interval in which the oscillation wavelength continuously increases at a constant change rate and the second oscillation interval in which the oscillation wavelength continuously decreases at a constant change rate. FIG. 42 shows a temporal change in the oscillation wavelength of the semiconductor laser 201. Referring to FIG. 42, reference symbol P1 denotes the first oscillation interval; P2, the second oscillation interval; λa, the minimum value of the oscillation wavelength in each interval; λb, the maximum value of the oscillation wavelength in each interval; and T, the period of a rectangular wave.
The laser light emitted from the semiconductor laser 201 is focused by the lens 203 and strikes the measurement target 210. The light reflected by the measurement target 210 is focused by the lens 203 and strikes the semiconductor laser 201. The photodiode 202 converts an optical output from the semiconductor laser 201 into a current. The current/voltage conversion amplifier 205 converts an output current from the photodiode 202 into a voltage. The signal extraction circuit 206 calculates the second-order differential of the output voltage from the current/voltage conversion amplifier 205. The counting circuit 207 counts the number of MHPs contained in the output voltage from the signal extraction circuit 206 in each of first and second oscillation intervals P1 and P2. The computing device 208 calculates the distance to the measurement target 210 and the speed of the measurement target 210 on the basis of the minimum oscillation wavelength λa and maximum oscillation wavelength λb of the semiconductor laser 201, the number of MHPs in the first oscillation interval P1, and the number of MHPs in the second oscillation interval P2.
The distance/speed meter disclosed in reference 4 can simultaneously measure the distance to a measurement target and the speed of the measurement target. In order to measure a distance and a speed, this distance/speed meter, however, needs to count the number of MHPs at least three times in, for example, a first oscillation interval t−1, second oscillation interval t, and first oscillation interval t+1. This meter requires a long period of time for measurement.